1. Field of the Invention
The present invention relates generally to fiber/resin composites and to methods of making such composites. In a specific aspect, the present invention relates to resin articles comprising arrays of continuous filaments, such as are formed by filament winding, prepregs and the like.
2. Description of the Related Art
In the field of composite materials, a variety of fabrication methods and techniques have come into usage for producing fiber-reinforced resin matrix materials. Continuous filament processes have evolved which are adapted to automated production of filament-reinforced resin articles. The continuous fiber processes include filament winding, wherein the filament, in the form of discrete strands or roving, is coated with a resin, then wound on a mandrel at a predetermined angle and winding thickness to yield composite articles having high strength when the resin borne on the filament is cured.
In order to have commercial utility the polymeric resins employed in filament winding operations must exhibit low initial viscosity and long pot-life in the process systems in which they are employed. Low viscosity is required in order that deposition of the resin on the filament be highly uniform in character, as is required to achieve substantially uniform properties in the final product article. If viscosity changes appreciably during the filament winding operation, the applied resin thickness may change significantly, resulting in localized stresses or discontinuities in the final product article, product articles which are not within required dimensional tolerance specifications, and inadequate curing of the resin. In addition, the tensional forces on the resin impregnated filaments being processed will significantly increase as the resin viscosity increases, to such extent that the filament becomes highly susceptible to snapping, i.e., tensionally breaking.
Long pot-life of the resin is particularly necessary in filament winding operations where processing times may be on the order of hours. Since the resin is continuously being applied to the filament in these processes, the resin bath or other source of the resin must be continually replenished with resin coating material, and it is therefore necessary that the resin not "set up" or gel in the source bath or other source container and applicating means.
For example, in the fabrication of rocket motors, a resin-bearing filament is wound onto a solid rocket fuel body. In such applications, since the filament winding operation may take upwards of 6 hours and since viscosity must be substantially stable during this period, a long resin pot-life is essential, and consequently the filament wound body must be rotated until full cure of the resin is achieved, which in the case of conventional epoxy resins can range from hours (for heat cured resins) to days (for resins cured at ambient temperatures). Continuous rotation of the mandrel and filament winding is essential in such cases, since cessation of rotation would result in the viscous resin sagging and dripping under gravitational forces, resulting in a resin-rich lower portion of the product article and a resin-poor upper portion of the product. Accordingly, it is desirable to cure the fiber array quickly once it has been formed.
The difficulties inherent in balancing the properties of long pot-life and a quick and easily controlled cure have resulted in the development of numerous types of resins. And within each class of resin, attempts have been made to vary the conditions under which the resins will cure effectively. The standard resins which have been employed in continuous filament processes, as well as in other systems of fiber/resin composite manufacture, generally have deficiencies which have specifically limited their utility in these processes.
The epoxy resins form an extremely important and versatile class of resins. These resins exhibit excellent resistance to chemicals, will adhere to glass and a variety of other materials, show electrical insulation properties, and are relatively easy to use. Among the epoxy resins, systems employing epoxy compounds in conjunction with olefinically unsaturated compounds have found wide acceptance in the art. In particular, resins comprising epoxies and acrylates have been found to be especially useful. This class of resins includes blends of epoxies and acrylates ("epoxy/acrylate" resins) as well as compositions wherein the principal resin component is an acrylic acid-modified epoxy in which some or all of the epoxy groups have been consumed to produce unsaturated resins. Partially acrylated epoxies are occasionally identified as "dual-functional" compounds since they are designed to exhibit both epoxy and acrylate functional groups on the same molecule.
Within the aforementioned class of epoxy/acrylate systems, compositions have been generated which are adapted to various cure conditions. Such compositions have employed heat curing mechanisms, actinic radiation curing mechanisms, or a combination of both.
Heat curing alone has several disadvantages including reducing the viscosity of the resin, causing it to become more fluid and thereby making it more difficult to handle the article, as well as more difficult to achieve a product of isotropic character. In applications such as filament winding, this drop in viscosity results in resin drip, as previously mentioned. Yet heat curing of epoxy/acrylate systems is an effective and practical means of curing the resins to the fully hardened state that is the source of the resins' great utility.
The use of actinic radiation to cure or partially cure, i.e., gel the resin, can substantially increase the viscosity of the resin on the formed article. Actinic radiation generally cannot induce complete hardening of the resin and such systems usually employ a catalyst and/or a heat cure step to fully cure the resins.
The class of epoxy/acrylate resin systems includes an especially useful subclass of resin systems which include, as an additional polymerizing component, a cyanate ester resin. Traditionally, cyanate ester resin compositions have employed thermosettable cyanate resins in combination with epoxies, acrylates and a variety of heat activated catalytic agents. Cyanate ester resin compositions have been found to be useful in applications ranging from castings and laminates to filament winding operations.
The following patents illustrate the state of the art of cyanate ester resin compositions. As a general rule, all cyanate ester resin compositions employ organic compounds having at least one cyanate ester functionality. These resin compositions are generally curable by heat and, as a result, include thermally activated catalytic agents such as amines, anhydrides, phenols, and organo-metal compounds.
U.S. Pat. No. 4,254,012 to Green discloses cyanate ester resins curable by a first UV light exposure and a second heat treatment. The compositions include polymeric cyanate ester resins; photopolymerizable compounds, such as acrylate esters; and heat activated catalysts such as Lewis acids, proton acids, bases, e.g., trimethylamine, phosphorous compounds, e.g., tributylphosphane, and organo-metals, e.g., zinc octanoate. The compositions also include a photoactivated polymerization catalyst such as an organic peroxide, a benzophenone, an acetophenone or other related photoinitiator or photosensitizer. The Green compositions are useful as coatings and may be adapted to filament winding.
U.S. Pat. Nos. 4,546,131, 4,559,399, 4,611,022 to Hefner, Jr., disclose thermosettable cyanate ester resin compositions comprising cyanate esters, epoxy compounds, and acrylates. These thermosettable composition also disclose thermally activated catalysts such as organic peroxides, various bases, "onium" salts, and trimerization catalysts such as organo-cobalt compounds. The Hefner, Jr. compositions may be adapted to filament winding applications. These compositions are not, however, adapted to UV light curing processes. Nor are the disclosed organic peroxides described as useful as heat-activated catalysts for improving the resistance of the compositions to dripping during heating.
The European Patent Application No. EP 266,986 to Amoco Corp. describes thermally cured resin compositions including cyanate esters, epoxy resins, and a thermoplastic polymer. Preferred cyanates are the cyanates of bisphenols. Preferred epoxies are polyglycidyl derivatives of phenols, novolaks, amines, and esters of carboxylic acids. The thermoplastic polymer may be either a polyacrylate or a polyetherimide. The compositions may also include bismaleimides for further polymerization with the resins. Thermally activated catalysts and accelerators described as useful for the resins include tertiary amines, phenols and transition metal organo-metal complexes. When bismaleimides are used, organic peroxides are described as preferred catalysts, but no disclosure is given regarding any class of peroxides useful for preventing drip during heat curing. The Gardner et al. resins may be adapted to filament winding. The Gardner et al. compositions, however, do not include acrylate type compounds and are not adapted to actinic light curing processes.
European Patent Document No. EP 347,800 to Mitsubishi Gas Chemical Co. Inc. discloses cyanate ester resin compositions including at least one acrylic resin, a saturated polyester or epoxy resin, an aromatic polycyanate, and a curing catalyst. Preferred epoxies include bisphenol type epoxies. The disclosed catalysts include organic peroxides, tertiary amines, phenols, anhydrides, and organo-metal salts as the preferred salts. The Mitsubishi compositions are described for coatings applications, but are not described as applicable to filament winding applications. Furthermore, the Mitsubishi compositions are not adapted to actinic radiation curing processes.
PCT Document No. WO 92/03516 to 3M Company describes single or dual cured adhesive compositions comprising at least one cyanate ester, a thermoplastic polymer, and an organo-metallic catalyst. The compositions may also include a silane coupling agent having an epoxy, glycidoxy, acrylic, amino or other hydrolyzable group. Preferred cyanates include cyanates of bisphenols and novolaks. Preferred thermoplastic polymers include polyesters, polyimides, and polyvinyls, including polyacrylates. The organo-metal catalysts disclosed in this document may be either heat activated or actinic light activated. The 3M patent document does not, however, disclose the use of an ultraviolet sensitive photoinitiator. Nor does the 3M patent document include epoxy type compounds except for the linkage of epoxy functionalities to the silane agents used for coupling the resin to a substrate. These compositions also are not adapted to filament winding operations.
A publication by Ising et al., entitled "Cyanate Cured Behavior and the Effect on Physical and Performance Properties" Third International SAMPE Electronics Conference, 360-370 (1989), discloses thermally cured resin compositions comprising cyanate esters, a bisphenol A epoxy resin, and an organo-metal catalyst. The Ising et al. compositions do not include acrylates and are not adapted to actinic light curing processes, having no actinic light sensitive component. These compositions also are not adapted for filament winding, but are described as useful for castings and laminates.
An anonymous publication entitled "Uncured Resin Mixture of Cyanate Ester Resin and Epoxy Resin Modified with Particulate Elastomer and Polymeric Material Prepared Therefrom" (1991), discloses thermosetting resin compositions including a polyaromatic cyanate ester, an epoxy resin, such as DGEBA, and an insoluble elastomeric particulate. The insoluble elastomeric particulate may include polyacrylates dispersed in a liquid diglycidyl ether of bisphenol A (DGEBA). Curing agents for these compositions include organo-cobalt compounds. These compositions are not adapted to actinic light curing processes and contain no photoinitiators. These resins may be reinforced with fibers, but are not described as useful for filament winding applications.
A publication by Shimp et al., entitled "Cyanate Ester-Cured Epoxy Resin Structural Composites" 37th International SAMPE Symposium, 293-305 (1992), provides a theoretical description of reaction pathways of cyanate-epoxy resin compositions. Shimp et al. also describe preparation methods for cyanate-epoxy compositions. Useful epoxies are described as including DGEBA, and the disclosed cyanates include bisphenol dicyanates. Disclosed catalysts include organo-titanate and organo-cuprate compounds. The Shimp et al. publication discloses resins that are curable by heat, but does not disclose any compositions curable by actinic radiation. The compositions of the Shimp et al. publication are described as adapted to various processes including filament winding.
Therefore, it would be a significant advance in the art to provide resin compositions and methods of using such compositions capable of overcoming the above-described difficulties associated with filament winding processes, in a manner which would obviate the use of additional curing steps and long rotation periods heretofore necessary to obtain quality composites having uniform characteristics.
The present invention solves the disadvantages inherent in the prior art by providing compositions that maintain stable low viscosities for a significant period of time, such that commercial filament winding processes are practicable. The compositions of the present invention also exhibit relatively high glass transition temperatures and are intended to be useful in high temperature applications. Unexpectedly, the resin compositions of the present invention allow uniform properties of the cured product to be obtained without drip or excessive flow of the resin during the heat-cure stage.
Accordingly, it is a purpose of the present invention to provide an improved process for forming fiber/resin composites.
It is a further purpose of the invention to provide an improved process for filament winding which overcomes the above-described deficiencies of the prior art practice of these processes.
It is another purpose of the invention to provide filament wound articles which are readily and economically formed, and which are rapidly processed for subsequent handling, packaging, or other processing operations.
Other purposes and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.